Distance relays are typically used to protect power transmission lines by detecting short circuit faults on the line and thereafter initiating the tripping of circuit breakers associated with the particular portion of the line covered by the relay.
A transmission line has a known impedance, which increases with the length of the line. A distance relay has a pre-established impedance setting, which determines the size of the relay's impedance characteristic, which is typically in the form of a circle in the impedance plane, and which is matched to the length of that portion of the line covered by the relay. The relay is capable of rapidly detecting faults on the transmission line, indicated by a drop in impedance of the line, by detecting when the impedance of the line is inside the impedance characteristic of the relay, i.e. inside the impedance plane circle.
The load which is serviced by the transmission line also appears to the distance relay as an impedance. The load impedance decreases as the load increases in normal operation of the power system. Typically, the load impedance remains large enough that it does not impinge on, i.e. "encroach" upon, the impedance characteristic (the circle) of the relay. In certain situations, however, the load is large enough (and hence the load impedance small enough) that it does overlap the relay characteristic. This is referred to generally as load encroachment. If it occurs, the distance relay will detect the reduced load impedance as being within the characteristic circle, and, not knowing that the reduced impedance determination is actually load, will identify it as indicating a fault condition on the line, and will trip the circuit breaker associated with that portion of the line, disrupting service unnecessarily to the heavily loaded line. This of course is highly undesirable, since no fault condition is in fact present on the line, i.e. it is a false trip, which is undesirable at any time, and since the false trip occurs at a very inconvenient time in the operation of the power system, when the demand for power is very high.
The conventional, universal solution to load encroachment is to modify the relay characteristic in some manner to exclude the load from the coverage of the characteristic. One approach is to use a mho relay element, which has a somewhat different reach than a conventional impedance circle. This sometimes will be sufficient to avoid the load. In those situations where the load still encroaches upon the mho characteristic, however, other more specialized techniques are necessary. In one technique, a portion of the mho circle is cut off by a blinder element. In another technique, the mho circle is specially configured to avoid the encroaching load pattern by using multiple circles. In still another technique, fancy patterns are used to avoid the load.
All of the conventional techniques attempt to shape the impedance characteristic to avoid the load. There are disadvantages to this approach, primarily in the resulting desensitization of the relay to faults which appear outside of the modified characteristic circle but which would have otherwise been inside the circle. It is difficult to match the characteristic to the load with any precision. Also, the more complex impedance characteristic shapes are relatively hard to generate by an operator through the relay settings. There is a relatively complex relationship between the relay settings and the transmission line loading conditions, which contributes to this problem.